In this paper, the effect of thermal expansion in ohmic metals during ohmic annealing was studied to understand the mechanism of n-type ohmic formation in AlGaN/GaN heterostructures. We measured ohmic contact resistance as a function of annealing temperature for metal stacks composed of Cu/Al/Mo/Au, Ni/Al/Mo/Au, and Ti/Al/Mo/Au. Specific contact resistivity values of lower than 10−5 Ωcm2 were, for the first time, achieved for both Cu/Al/Mo/Au and Ni/Al/Mo/Au. The minimum contact resistance for Cu/Al/Mo/Au, Ni/Al/Mo/Au, and Ti/Al/Mo/Au were 0.36 Ωmm, 0.39 Ωmm, and 0.26 Ωmm, respectively.